Decreased H2 sensitivity in optical fiber

ABSTRACT

Disclosed is a method of making a hydrogen resistant optical waveguide fiber. The soot preform is heated and immersed in a metal halide gas. A reduced metal species is thus incorporated into the glass soot prior to sintering or consolidation of the soot preform. A hydrogen absorption band around 1530 nm is substantially eliminated from waveguides made from a precursor gas treated preform.

[0001] This application claims the benefit of U.S. patent applicationSer. No. 09/116,095 filed Jul. 15, 1998, the benefit of priority ishereby claimed.

FIELD OF THE INVENTION

[0002] The invention relates to a method for decreasing the sensitivityof optical waveguide fiber to hydrogen. In particular, the methodmarkedly reduces hydrogen induced attenuation in single mode opticalwaveguide fiber in a wavelength band centered about 1530 nm.

BACKGROUND OF THE INVENTION

[0003] Hydrogen can react with defects in silica based optical waveguidefibers to form unwanted signal absorption bands. A number of strategieshave been developed to avoid the incorporation of hydrogen into thewaveguide fiber, including sealed cables, hermetically coated waveguidefiber, and optical fiber cabling materials or coatings which act ashydrogen getters.

[0004] An example of the hydrogen getter approach is found in U.S. Pat.No. 5,596,668, DiGiovanni et al. ('668). The species for gettering orbonding with hydrogen, in this case a metal, is placed in the clad layerof the waveguide fiber. Diffusion of hydrogen into the light carryingportion of the waveguide is reduced and the waveguide is said to behydrogen resistant. Care must be taken to prevent inclusion of thegetter species into the core region and the part of the clad layeradjacent the core region. These regions carry the signal light and thepresence of getter material in the regions would cause unacceptablesignal attenuation. The '668 patent at column 3, II. 65-67 and in FIGS.2, 3, and 4 makes clear the getter material must be located away fromthe light carrying part of the waveguide. This limitation together withthe fact that hydrogen diffusion is not completely eliminated makes thisapproach less than optimum.

[0005] Providing the waveguide with a hermetic coating does essentiallyeliminate hydrogen induced attenuation. However, the application of thecoating involves an additional process step which adds considerable costin terms of raw materials, equipment, and manufacturing rate. Extrameasurement steps to insure the hermeticity of the coating are alsorequired.

[0006] An alternative getter method is one in which the getter materialis incorporated in the waveguide polymer coating or in the materialswhich make up the cable. Such alternatives involve additional expenseand the materials must be such that they will not degrade or otherwiseleave the host material for the life of the waveguide, which is usuallyestimated in decades.

[0007] U.S. Pat. No. 4,125, 388, Powers ('388 patent), discloses andclaims a method for making high purity optical waveguides, especiallywaveguides having very low concentrations of water. The inclusion ofwater in the silica-based glass matrix gives rise to broad absorptionbands in wavelength ranges otherwise well suited to signal transmission.The '388 patent discloses and claims a method for making very low waterwaveguides by removing water from the soot preform during the step inwhich a soot preform is heated to fuse the soot particles into a glass.The '388 patent discloses the use of C1 ₂ gas as a drying agent. The Cl₂may be fed directly to the preform or a metal halide gas, such as GeCl₄and SiCl₄, may be used together with an oxidizing agent to produce Cl₂in the vicinity of the preform. The drying is carried out within atemperature range in which the soot will fuse into a dense glass.

[0008] In contrast to this drying method, the method of the presentinvention includes a step which precedes the drying step and which iscarried out at a temperature below that at which the preform will beconsolidated.

[0009] Thus there is a need in the waveguide fiber industry for a methodof eliminating hydrogen sensitivity which method:

[0010] fits readily into the flow of the existing waveguide fibermanufacturing process;

[0011] does not cause a marked reduction in manufacturing rate;

[0012] is simple and cost effective; and,

[0013] is built into the glass itself and so is reliable over the lifeof the waveguide.

SUMMARY OF THE INVENTION

[0014] The novel method and the resulting waveguide fiber derivedtherefrom disclosed and described here, meet the need for a low costhydrogen resistant waveguide which has excellent long term reliabilityand which overcomes the deficiencies in the art noted above.

[0015] One embodiment of the invention relates to a method of making ahydrogen resistant optical waveguide fiber. A soot preform is fabricatedby any one of several methods known in the art such as outside vapordeposition or axial vapor deposition. The method can be extended toinclude a modified inside vapor deposition preform manufacturing methodby lengthening the time between soot deposition and soot consolidationor by including an excess of GeCl₄ or SiCl₄ with regard to oxygen. Byany of several methods known in the art, at least a part of the centralcore region of the soot preform is made to have a refractive indexhigher than at least a part of the surrounding cladding glass layer.These methods can include co-deposition of a soot in the central regionto raise the refractive index, co-deposition of a soot in thesurrounding layer to lower the index, or treatment of the soot of eitherregion with index modifying gases such as fluorine. Thus, modificationof the refractive index can be accomplished during soot deposition orafter soot deposition but prior to soot consolidation.

[0016] In one preferred embodiment, the method of deposition is theoutside vapor deposition process, and GeCl₄ or SiCl₄ are employed todeposit a GeO₂ doped SiO₂ core region onto a bait rod. This ispreferably followed by deposition of at least a minimal amount of a SiO₂cladding region (additional cladding may also be deposited now or at alater stage, if desired). The bait rod is them removed, and theresultant soot preform can be treated in accordance with the invention.In one such embodiment, a metal halide gas (e.g. GeCl₄) is flowed aroundthe soot preform (and through the hole left by removal of the bait rod,if one was employed to make the preform). Note that in the novel methoddescribed herein, the metal halide gas is preferably in excess relativeto oxygen. This is in contrast to the smaller metal halide to oxygenratio which is advantageous in a drying process.

[0017] In one embodiment of the present novel method, the soot preformis heated to a temperature greater than about 800° C. but less than thesoot consolidation or sintering temperature. A metal halide gas which isa precursor of a glass forming metal oxide is then caused to flowthrough or about the hot, porous soot, preferably at a flow rate whichis not less than about 0.2 standard cubic centimeters per minute (sccm)per 100 grams of soot glass. As is known in the art, the succeedingprocess steps can include sintering the soot to form a clear glass body,adding additional overcladding if needed or desired, and collapsing orsintering it, and then drawing a waveguide fiber from the resulting drawpreform. A flow rate of about 1 sccm or more per 100 g of soot glass ispreferred, although a flow rate as low as 0.2 sccm/100 g is effective toimprove hydrogen resistance. There is essentially no process reason forplacing an upper limit on the flow rate. Thus the upper limit isdictated by material cost and equipment capability. A rate of 1.0sccm/100 g is well within the capability of the equipment used to dryand sinter the soot preform.

[0018] The action of the metal halide gas on the soot preform istypically substantially complete in 1 hour. Variability in soot densitymay require that soot preform be exposed to the metal halide gas forlonger time periods or shorter time periods may be effective. A range ofabout 0.5 to 10 hours has been found to cover the normally encounteredrange of soot densities and temperatures. In a preferred embodiment ofthe method, the soot preform is held near in the range of about 1000° C.to 1150° C. during immersion in the metal halide gas flow. However, themethod is effective at least to temperatures as high as 1250° C.

[0019] The method works well when the index increasing core dopant isgermania, although the method will be effective for other core glassdopants. Typical metal halide gases which may be used in the methodinclude GeCl₄ and SiCl₄.

[0020] In an alternative embodiment, the same effect can be achieved byutilizing a soot deposition process, employing a metal halide precursor(GeCl₄) during soot deposition, and employing less than a stoichiometricamount of oxygen in the reaction chamber. In this manner, incorporationof an adequate amount of reduced Ge can be supplied outside the GeO₂doped core.

[0021] A second aspect of the invention is a hydrogen resistant opticalwaveguide fiber made using the novel method.

[0022] A third aspect of the invention is a soot preform and a method ofmaking a soot preform which is a precursor of a hydrogen resistantwaveguide fiber. The method for making the soot preform includes thesteps of depositing soot on any of several suitable soot collectingtargets known in the art such as a bait rod of carbon, silica, oralumina, or on the inside or outside of a silica based glass tube. Thesoot comprises a silica layer and a core region of silica doped with anindex raising material such as germania. Before sintering, the sootpreform is heated and treated with a metal halide gas as before.

[0023] A fourth aspect of the invention is an optical waveguide fiberwhich contains reduced metal species (e.g. reduced germanium) in thecore region or in the clad region immediately adjacent the core. Theclad region immediately adjacent the core region is a ring of thickness5 to 10 μm surrounding the core region.

[0024] The presence of such reduced metal species is the result of thetreatment of the soot preform with a metal halide gas. The reduced metalspecies may be detected and quantified by any of a number of methods inthe art. For example, the presence of reduced Ge may be quantified bymeasuring the absorption by the waveguide or waveguide glass preform oflight having a wavelength near 240 nm. Absorbance is equal to(1/t)log(I_(o)/I), where t=sample thickness, I_(o)=incident intensity,and I=transmitted intensity. In the case of a glass made from agermanium halide gas treated soot preform, it has been found that anabsorbance of not less than about 0.3/mm of 240 nm light, at a radialpoint located outside the GeO₂ doped region of the core, is indicativeof hydrogen resistant glass. In one preferred embodiment, this region ispresent at or about halfway through the thickness of the adjacent cladring, or greater than 1, more preferably greater than 3 microns outsidethe GeO₂ doped SiO₂ region. More preferably, the absorbance at thiswavelength is less than about 0.2/mm. That is, sufficient reduced Ge ispresent in the glass to make a waveguide fiber which is hydrogenresistant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is an illustration of a soot preform suspended in a furnacethrough which a metal halide gas may be flowed.

[0026]FIG. 2a is a cross section of a soot preform.

[0027]FIG. 2b is a cross section of a waveguide fiber or a draw preform.

[0028]FIG. 3 is a chart of weight percent GeO₂ versus radial position ina glass preform.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The novel method of making a hydrogen resistant optical waveguidefiber may be practiced using the soot deposition equipment, preformdrying and consolidation equipment, and preform drawing equipment whichis used in any of several alternative manufacturing processes known inthe art. FIG. 1 shows a soot preform 6, made by any of severalalternative processes, suspended by mechanical means 2 in a furnace 4.

[0030] Inlet 8 and outlet 10 provide a means of flowing a gas over thesoot preform prior to sintering. It is believed that the flow of metalhalide gas in this part of the waveguide manufacturing process(consolidation) is most effective in introducing reduced metal speciesinto the soot preform. Further, it is believed preferable that the timelapse between metal halide gas flow and soot sintering be held to aminimum, and can, for example, include flowing the metal halide gasright up until the time that consolidation occurs or is complete . Thesebeliefs are explanatory in nature and are not meant to limit theinvention in any way. It will be understood that any of severalalternative strategies may be used for flowing the metal halide gas intothe furnace. For example, the inlet could be 10 and the outlet 8 inFIG. 1. In some instances the metal halide gas may be introduced intoone or more tubular formations extending horizontally or verticallythough the soot preform.

[0031] Handle 44 is suspended from a support tube 46 for insertion intoconsolidation furnace 15. Handle 44 comprises glass tube 45 having aflared joint 48 at its upper end and an annular enlargement 49 spacedfrom the joint 48. Support tube 46 has a slotted handle formed in theend thereof. One side of end region 47 of tube 46 is removed to acceptthe upper end of handle 44, enlargement 49 resting on slotted base 50 asthe adjacent section of tube 45 is inserted into slot 51. At the end ofgas conducting tube 53 is a ball joint 52 which fits into cavity 54 ofjoint 48.

[0032] The soot preform is preferably exposed to the metal halide gasfor a time and temperature which is sufficient to result in a fiberwhich exhibits decreased sensitivity to hydrogen, e.g. a fiber whichexhibits less than 0.05 dB/km increased attenuation at 1530 nm afterexposure to a 1% hydrogen atmosphere for 6 days, more preferably lessthan than 0.03 dB/km increased attenuation at 1530 nm after exposure toa 1% hydrogen atmosphere for 6 days, most preferably less than than 0.01dB/km increased attenuation at 1530 nm after exposure to a 1% hydrogenatmosphere for 6 days. The fact that such fibers, which have a decreasedsensitivity to hydrogen, are possible without having to apply a hermeticcoating to the fiber is a tremendous advantage over previous fibers. Thecross section of a soot preform, FIG. 2a, shows the core soot 11 and theadjacent clad soot layer 12. This porous body made up of core and cladsoot is heated in a furnace and immersed in the metal halide gas flow.Once the treatment with the metal halide gas is completed, the sootpreform may be sintered to form a glass body and an additional layer ofcladding glass 14 may be applied. Typically the extra clad layer issleeved over or deposited upon the sintered preform. The resulting drawpreform is illustrated in FIG. 2b which shows the core region 10, theadjacent clad layer 12 and the outer clad layer 14.

[0033] The effect of flowing a metal halide gas over the heated sootpreform during consolidation is illustrated in FIG. 3. FIG. 3illustrates weight percent germania versus radial position in a sinteredpreform for both a metal halide gas (in this case GeCl₄) treatedpreform, curve 16, and an untreated preform, curve 18. The excess weightpercent GeO₂ present in curve 16 is indicative of additional Ge in theglass matrix. The x-axis is divided into arbitrary units of length. Theportion of the preform illustrated in FIG. 3 is only the portion that islocated at the interface between the core glass region and the cladglass layer. The excursion of curve 16 above curve 18 indicates thatgermanium from the GeCl₄ gas flow has been taken up into the preformmatrix. The 240 nm light absorbence measurement confirms that the Ge isin its reduced form.

[0034] While not wanting to be bound by theory, applicants believe thatthe mechanism which makes the resulting waveguide fiber hydrogenresistant is as follows. Defects exist in the glass matrix which are dueto the presence of excess oxygen. Treating the soot preform with a metalhalide (M Clx, where M stands for metal and x depends upon the metalvalence) causes metal atoms to be inserted into the matrix, eliminatingsurplus oxygen and the associated defect. Thus, treating the sootpreform with a pre-selected gas substantially eliminates bonds prone toforming draw induced or otherwise stress induced defects. This modelfits well with the behavior of atoms in a glass matrix and does explainpertinent hydrogen absorption bands observed in testing. It will beunderstood however that the invention is in no way limited by this modeland does not depend upon the correctness of the model.

[0035] The invention is further illustrated by the following exampleswhich are meant to be illustrative and in no way limiting.

EXAMPLE 1 (COMPARATIVE) Heating without a Precursor Gas

[0036] A soot preform was fabricated using an outside vapor depositionmethod in which glass soot was deposited upon a bait rod. The coreregion comprising silica and germania was deposited. A layer of silicawas deposited about the core region. The bait rod was removed and thesoot preform was placed in a furnace and heated to 1000° C. For a 1 hourtime period, 20 slpm of He was flowed around the preform and 0.7 slpm ofHe was passed through the preform center opening. Then for a 3 hour timeperiod, a flow of 0.07 slpm of Cl₂ was added to the flow of He gas inthe preform center opening. The Cl₂ flow was stopped and the furnacetemperature was raised and the preform was sintered to form a clearglass body. The sintering process is known in the art and will not befurther described here. The nominal diameter of the sintered glass bodywas 7 mm. The portion of the sintered glass body characterized as thewaveguide cane had a nominal diameter of 3.5 mm.

[0037] The absorption of 240 nm light at three positions in the silicalayer was measured to estimate the amount of reduced Ge incorporatedtherein. The measurements were:

[0038] near the core region clad layer interface—0.27;

[0039] 0.75 mm further out in the clad from the core—cladinterface—0.09; and,

[0040] 1.25 mm further out in the clad from the core interface—0.03.These readings indicate the diffusion of Ge from the core region is notappreciable a few millimeters from the core region.

[0041] A waveguide fiber made using this preform was tested in a 1%hydrogen atmosphere for 6 days. The pressure in the testing chamber was1 atmosphere and the chamber was held at room temperature. The increasein attenuation at 1530 nm was measured to be 1.450 dB/km.

EXAMPLE 2 Heating with a Metal Halide Gas

[0042] A soot preform was fabricated using a process identical to thatin the example above, except that the preform was treated with metalhalide gas in accordance with the invention.

[0043] The soot preform was placed in a furnace and heated to 1000° C.For a 1 hour time period, 20 slpm of He was flowed around the preformand 0.7 slpm of He was passed through the preform center opening. Thenfor a 3 hour time period, the center flow was maintained and a flow of 1sccm/100 g of GeCl₄ was added to the 20 slpm flow. The GeCl₄ was stoppedand the furnace temperature was raised and the preform sintered to forma clear glass body using a process identical to that in the exampleabove and resulting in an essentially identical sintered preformgeometry.

[0044] The absorption of 240 nm light at the same three positions in thesilica layer as before, was measured to estimate the amount of reducedGe incorporated therein. The measurements were:

[0045] near the core region interface—2.1;

[0046] 0.75 mm further out from the core interface—1.8; and,

[0047] 1.25 mm further out from the core interface—1.2. This indicatesthat additional Ge has been incorporated into the metal halide gastreated preform.

[0048] A waveguide fiber made using this preform was tested in a 1%hydrogen atmosphere for 6 days. The pressure in the testing chamber was1 atmosphere and the chamber was held at room temperature. The increasein attenuation at 1530 nm was measured to be 0.004 dB/km which is verynear the noise floor of the measurement. The incorporation of reduced Geinto the preform in the clad layer adjacent the core region was shown tobe effective to essentially eliminate hydrogen sensitivity in awavelength band centered at 1530 nm.

[0049] Although specific embodiments of the invention have herein beendisclosed and described, it is understood that such detail is solely forthat purpose and variations can be made without departing from thespirit and scope of the invention which is defined by the followingclaims.

What is claimed is:
 1. A hydrogen resistant optical waveguide fiber comprising: a central core region surrounded by and in contact with a clad region, both of said core region and said clad region comprising a silica based glass; whereby, said fiber exhibits less than 0.05 dB/km increased attenuation at 1530 nm after exposure to a 1% hydrogen atmosphere for 6 days.
 2. The fiber of claim 1, wherein said fiber exhibits less than 0.03 dB/km increased attenuation at 1530 nm after exposure to a 1% hydrogen atmosphere for 6 days.
 3. The fiber of claim 1, wherein said fiber exhibits less than 0.01 dB/km increased attenuation at 1530 nm after exposure to a 1% hydrogen atmosphere for 6 days.
 4. A method of making a treated soot preform which is a precursor of a hydrogen resistant waveguide fiber, comprising the steps of: fabricating an optical fiber preform comprising a central core region surrounded by and in contact with a clad region; and, during or after said fabricating step, exposing said preform to a metal halide gas in an atmosphere and for a time and temperature which is sufficient to treat said preform so that, when said preform is employed in a fiber draw process for making an optical fiber, the resultant fiber exhibits less than 0.05 dB/km increased attenuation at 1530 nm after exposure to a 1% hydrogen atmosphere for 6 days.
 5. The method of claim 4, wherein the central core region and the clad region of said preform in said exposing step are both comprised of silica based soot.
 6. The method of claim 5, wherein, more preferably less than than 0.03 dB/km increased attenuation at 1530 nm after exposure to a 1% hydrogen atmosphere for 6 days.
 7. The method of claim 5, wherein said step of exposing comprises heating the soot preform to a temperature greater than 800° C. but less than the sintering temperature of both the core region soot and the clad region soot.
 8. The method of claim 5, wherein said exposing step comprises maintaining the soot preform at a substantially constant temperature.
 9. The method of claim 5, wherein said exposing step comprises flowing said metal halide gas around or through said soot preform.
 10. The method of claim 5, further comprising sintering the preform to form a clear glass body.
 11. The method of claim 10, further comprising providing additional cladding soot material over the silica layer of the clear glass body to form a draw preform and drawing an optical waveguide fiber from said draw preform.
 12. The method of claim 5, wherein the total metal halide gas flow around or through the soot preform in said exposing step is not less than about 0.2 sccm/100 grams of glass.
 13. The method of claim 12, wherein the gas flow is not less than 1.0 sccm/100 grams-of glass.
 14. The method of claim 5, wherein the time duration of said exposing step is in the range of about 0.5 to 10 hours.
 15. The method of claim 5, wherein the temperature in said exposing step is less than about 1250° C.
 16. The method of claim 5, wherein the temperature in said exposing step is in the range of about 1000° C. to 1150° C.
 17. The method of claim 5, wherein said core region comprises a region which is comprised of germania soot which is co-deposited with silica soot.
 18. The method of claim 17, wherein said metal halide gas in said exposing step is selected from the group consisting of GeCl₄ and SiCl₄.
 19. The method of claim 5, wherein said metal halide gas in said exposing step is selected from the group consisting of GeCl₄ and SiCl₄.
 20. A hydrogen resistant optical waveguide made using the method of claim
 4. 21. The method of claim 4, wherein said exposing step occurs during said fabricating step, and said method further comprises depositing SiO₂ soot, along with said metal halide, in an atmosphere having less than a stoichometric amount of oxygen.
 22. The method of claim 21, wherein said metal halide is GeCl₄.
 23. The method of claim 22, wherein said SiO2 soot is deposited via chemical vapor deposition using SiCl₄.
 24. A hydrogen resistant optical waveguide fiber comprising: a core glass region surrounded by and in contact with a clad glass layer, the respective core and clad glass having a refractive index profile and at least a portion of the core glass region having a refractive index which is higher than the refractive index of at least a portion of the clad glass layer; in which at least a portion of the core glass region or a portion of the clad glass region adjacent the core glass region contains a reduced metal species.
 25. The hydrogen resistant waveguide of claim 24 in which the reduced metal species is selected from the group consisting of Ge and Si.
 26. The hydrogen resistant waveguide fiber of claim 24 in which the reduced metal species is Ge and the absorbence of 240 nm light directed along the waveguide axis is not less than about 0.2/mm when the radial position of the light is such that the light is absorbed by the clad layer adjacent the waveguide core and no more than 5 to 10 μm from the periphery of the core. 